1. Field of the Invention
The invention relates generally to the field of marine seismic survey apparatus and methods. More specifically, the invention relates to structures for marine seismic streamers that have reduced noise induced by effects of towing such streamers in the water.
2. Background Art
In a marine seismic survey, a seismic vessel travels on the surface of a body of water such as a lake or the ocean. The seismic vessel typically contains seismic acquisition control equipment, which includes devices such as navigation control, seismic source control, seismic sensor control, and signal recording devices. The seismic acquisition control equipment causes a seismic source towed in the body of water, by the seismic vessel or another vessel, to actuate at selected times. The seismic source may be any type well known in the art of seismic acquisition, including air guns or water guns, or most commonly, arrays of air guns. Seismic streamers, also called seismic cables, are elongate cable-like structures that are towed in the body of water by the seismic survey vessel or by another vessel. Typically, a plurality of seismic streamers is towed behind the seismic vessel laterally spaced apart from each other. The seismic streamers contain sensors to detect the seismic wavefields initiated by the seismic source and reflected from acoustic impedance boundaries in the subsurface Earth formations below the water bottom.
Conventionally, seismic streamers contain pressure-responsive sensors such as hydrophones, but seismic streamers have also been proposed that contain particle motion sensors, such as geophones, in addition to hydrophones. The sensors are typically located at regular intervals along the length of seismic streamers.
Seismic streamers also include electronic components, electrical wiring and may include other types of sensors. Seismic streamers are typically assembled from sections, each section being approximately 75 meters in length. A number of such sections are joined end to end, and can extend the assembled streamer to a total length of many thousands of meters. Position control devices, such as depth controllers, paravanes, and tail buoys are affixed to the streamer at selected positions and are used to regulate and monitor the movement of the streamer in the water. During operation, the seismic sources and streamers are typically submerged at a selected depth in the water. The seismic sources are typically operated at a depth of 5-15 meters below the water surface and the seismic streamers are typically operated at a depth of 5-40 meters.
A typical streamer section consists of an external jacket, connectors, spacers, and strength members. The external jacket is formed from a flexible, acoustically transparent material such as polyurethane and protects the interior of the streamer section from water intrusion. The connectors are disposed at the ends of each streamer section and link the section mechanically, electrically and/or optically to adjacent streamer sections and, therefore, ultimately link it to the seismic towing vessel. There is at least one, and are usually two or more such strength members in each streamer section that extend the length of each streamer section from one end connector to the other. The strength members provide the streamer section with the capability to carry axial mechanical load. A wire bundle also extends the length of each streamer section, and can contain electrical power conductors and electrical data communication wires. In some instances, optical fibers for signal communication are included in the wire bundle. Hydrophones or groups of hydrophones are located within the streamer section. The hydrophones have sometimes been located within corresponding spacers for protection. The distance between spacers is ordinarily about 0.7 meters. A hydrophone group, typically comprising 16 hydrophones, thus extends for a length of about 12.5 meters.
The interior of the seismic streamers is filled with a void filling material to provide buoyancy and desired acoustic properties. Most seismic streamers have been filled with a liquid core material, such as oil or kerosene. Such liquid-filled streamer design is well proven and has been used in the industry for a long time. However, there are disadvantages associated with using liquid as a core fill material. The first disadvantage is leakage of the liquid into the surrounding water in the event a streamer section is damaged. Such leakage may present an environmental problem. Damage can occur while the streamer is being towed through the water or it can occur while the streamer is being deployed from or retrieved onto a streamer winch on which streamers are typically stored on the seismic vessel.
A second disadvantage to using liquid-filled streamer sections is noise induced in the hydrophones generated by vibrations as the streamer is towed through the water. Such vibrations develop internal pressure waves that travel through the liquid in the streamer sections, such waves are often referred to as “bulge waves” or “breathing waves.”
Ideally, in a streamer moving at constant speed, all the streamer components including the jacket, the connectors, the spacers, the strength members, wire bundle, sensors and void filling material all move at the same constant speed and do not move relative to each other. Under actual movement conditions, however, transient motion of the streamers takes place, such transient motion being caused by events such as pitching and heaving of the seismic vessel, movement of the paravanes and tail buoys attached to the streamers, strumming of the towing cables attached to the streamers caused by vortex shedding on the cables, and operation of depth-control devices located on the streamers. Any of the foregoing types of transient motion can cause transient motion (stretching) of the strength members.
Transient motion of the strength members displaces the spacers or connectors, causing pressure fluctuations in the liquid void filling material that are detected by the hydrophones. Pressure fluctuations radiating away from the spacers or connectors also cause the flexible outer jacket to compress in and bulge out in the form of a traveling wave, giving the phenomenon “bulge waves” its name.
In addition, there are other types of noise, often called “flow noise”, which can affect the quality of the seismic signal detected by the hydrophones. For example, vibrations of the seismic streamer can cause extensional waves in the outer jacket and resonance transients traveling down the strength members. A turbulent boundary layer created around the outer jacket of the streamer by the act of towing the streamer can also cause pressure fluctuations in the liquid core-filling material. In liquid filled streamer sections, the extensional waves, resonance transients, and turbulence-induced noise are typically much smaller in amplitude than the bulge waves, however they do exist and affect the quality of the seismic signals detected by the hydrophones. Bulge waves are usually the largest source of vibration noise because these waves travel in the liquid core material filling the streamer sections and thus act directly on the hydrophones.
One approach to overcoming the disadvantages of liquid fill in streamers is to use a gel like fill made from curable polyurethane. Using a soft, flexible gel like material can also eliminate a substantial portion of the problem with “bulge waves”, but the so-called Poisson effect from the strength members can increase. Because of the relatively high tensile stiffness of the strength members, transients generally travel along the strength members at velocities near to or lower than that of the sound velocity in water, such velocities typically in the range of 1000 to 1500 meters per second. The actual velocity of transients along the strength members depends mainly on the elastic modulus of the strength member material and the tension applied to the streamer as it is towed in the water. The lower the elastic modulus the more compliant the streamer will be, and thus the more transient energy it will dissipate as heat and the less will pass through the strength member. Special elastic sections are normally placed at either end of a streamer cable to reduce the effects of transients.
A streamer based on a buoyancy fill material made from curable, polyurethane based gel will have a longitudinal wave that is carried through the strength members of the streamer. When the streamer is excited by transient motion, the wave typically will travel with a velocity of around 1250 meters per second. When the longitudinal waves travels through the streamer they elongate and contract the streamer and generate pressure variations in the gel. The pressure variations will be detected by the sensors (hydrophones) and this will result in noise in the detected seismic data. The noise is normally at frequencies below about 30 Hz. The main reason for the pressure variations is believed to be that the deformation of the jacket is not equal to the deformation of the gel and therefore this mismatch generates pressure variation.
There is still a need to further improve the attenuation of longitudinal waves transmitted through the strength members of marine seismic streamers.